How best to deploy Fusion drive + External (Thunderbolt 2) SSD

Discussion in 'iMac' started by camner, Feb 25, 2017.

  1. camner macrumors regular

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    Jun 19, 2009
    #1
    I am moving from a 2010 Mac Pro to a 2015 (current) 27" iMac with a 2TB Fusion drive. I also have an external (OWC Thunderbay) external drive enclosure.

    The Mac Pro was configured with an internal 500GB SSD boot drive. I kept large files and those not needing fast loading on an internal 7200rpm SATA drive.

    I can imagine a variety of ways of configuring the iMac + Thunderbolt 2 SSD combination, and I wonder if others have experience that might be useful. Here are two I came up with...
    1. Use internal 2TB Fusion drive as the boot drive, and place files benefiting from quick loading on the SSD in the TB enclosure. Obviously, this means that many apps and files in my user folder will be on the slower part of the Fusion drive (the mechanical part).
    2. Use the 500GB SSD in the Thunderbay enclosure as the boot drive, and place large files and those not needing fast loading on the internal Fusion drive.
    In general, an SSD is much faster than a mechanical drive. But I don't know how to compare the relative speeds of a Fusion drive that is internal to an SSD that is external, but in a Thunderbolt enclosure (that enclosure will also have 3 mechanical drives in addition to the SSD). I'm not sure I can really test throughput with something like Black Magic Speed Test, because I have no way of kno wing whether when it tests the Fusion drive it is reading/writing the 128GB SSD in the iMac or the mechanical drive part of the Fusion drive, right? (FWIW, right now I have about 300GB on the Fusion drive, and I ran Black Magic Speed test, and the Fusion drive blew away the external SSD! The Fusion drive scored about 1300 read/525 write, while the external Thunderbolt SSD did 385 read/340 write. This surprised me. I also don't know what kind of mechanical drive is in the 27" iMac (7200 rpm? 5400?) nor the kind of interface it has (thus I don't know its xfr rate).

    Thoughts?
     
  2. casperes1996 macrumors 65816

    casperes1996

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    #2
    You can also include the external inclosure in the Fusion Drive setup if you want. It only requires a bit of fiddling with the Terminal. Or create two Fusion Drives. the internal one, and a combo of the mechanical and SSD drives you have connected externally

    The mechanical in the iMac is a 7200RPM with a SATA III connection.
    Regarding your BlackMagic scores, OS X/macOS always leaves, I believe it's 4GB, may be more, free space available on the SSD portion for active application usage, such as writes here and now, or RAM paging. It'll then offload the written data to the HDD later if the files aren't often accessed. So your Black Magic scores are pretty much exclusively the SSD, they most likely a combination that's 98% SSD 2%HDD or something like that.
     
  3. camner thread starter macrumors regular

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    #3
    OK, so Other World Computing has two dual bay docks, one USB 3.1 at about $70 and the other Thunderbolt 2 at about $250. If physical spinning drives can only deliver a max of about 150 MB/s, and since USB 3.0 tends to max out at under 500 MB/s (out of a theoretical maximum of about 600, if I recall correctly), a single USB 3.0 cable shouldn't be saturated with data with both bays in use continuously. If I'm right about this, what is the point of a Thunderbolt 2 drive dock? The TB 2 data rate is far greater, but the physical disks can't transfer data to even fill a USB 3.0 connection, right?

    On the OWC site, under the TB dock, they write:
    Benchmarks
    • Single Drive
      • Thunderbolt: 522 MB/s Read and 477 MB/s Write
      • USB 3.0: 427 MB/s Read and 347 MB/s Write
    If a spinning drive can only deliver 150MB/s, then these benchmarks must only apply to SSDs, yes?

    Or am I totally missing something here?
     
  4. camner thread starter macrumors regular

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    #4
    So, as I thought, trying to assess the speed of a Fusion drive is pretty much hopeless, yes?

    Thanks for your help. I appreciate the additional thoughts, though I'm not sure I want to create a Fusion drive with a mixture of internal and external hardware.

    With respect to my two choices, given that one can't test the speed of the Fusion drive, can I safely assume that the speed of the external SSD with Thunderbolt 2 connection would be faster than the internal Fusion drive, thus leading to the conclusion that making the external SSD the boot drive is the better option?
     
  5. casperes1996 macrumors 65816

    casperes1996

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    #5
    Those benchmarks are clearly with SSDs, yeah. That said, there can still be a performance boost using TB with a spinning platter, vs. using USB 3. A very small one, but still a difference. And it comes down to overhead. The data bus itself isn't saturated in terms of bandwidth, but there's more than bandwidth to consider. Latency for instance.

    Depends what you mean by the speed of a Fusion Drive. Since it dynamically moves stuff around and whatnot, it won't really have "one speed", but rather the speed of the SSD (which is what you tested with Black Magic, probably + a little more), and the speed of the HDD. Depending on what you're accessing you'll experience the speed of whichever the data is on. For writes, you'll always experience the SSD's speed for the first, I believe, 4GB. Maybe more, but at least 4. That much is always reserved as free for newly written data, to later be moved to where appropriate. That means if you import 4GB of photos, whilst they're newly imported, they will be on the SSD only, and only after being left for a while or potentially rebooted, will they move to the HDD. The 4GB thing btw, is with the 128GB SSD in the original Fusion or the 2TB+ Fusion Drives. Not the ones with only 64GB of SSD. Don't know if it's different for those. I have a Fusion Drive myself, and honestly, 90% of the time it feels like it's a pure SSD machine. Like, you have so many files (or at least I do) that don't really see a benefit to faster storage, like small Pages files, songs in iTunes, Photos, where the HDD is fast enough for a good viewing experience, and so forth. For where it makes sense, the files are already on the SSD.

    Sure, mixing internal and external might not be optimal. But you can also keep the internal part the way it is, and still make a Fusion Drive of the external disks, so it'll automatically move data between external HDDs and external HDD depending on how often data is used, to optimise speed. It works like a JBOD array, but automatically managing data between drives of different speeds.

    The external SSD will be faster compared to the HDD in the Mac, but by the sounds of it slower than the SSD, so it would depend on the data. For pure boot times, the internal would be faster, since it's a faster SSD, and the system itself will always reside on the SSD only. If you have more than 128GB of data that requires fast storage it's better placed on the SSD itself - which also allows you more reliability in terms of speed, knowing your data won't get moved to slower storage. However, remember that the Fusion Drive works on a block level, not a file level, so for me, Final Cut itself might not be placed wholly on the SSD, but the most important parts of the app probably are, and the ones that only get loaded up once, when you launch the app, and then just stay in memory, they'll probably be on the HDD.
     
  6. bplein macrumors 6502

    bplein

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    #6
    I own a late 2012 27" iMac, it came with a 1TB Fusion Drive.

    I use a 512GB SSD in an external TB enclosure as my boot/Applications drive, as well as my home directory.

    I have upgraded the internal HDD to 2TB and re-fused it with the 128GB internal SSD, resulting in a 2TB Fusion Drive. I use this for media (lots of photos in my Lightroom repository, iTunes, etc.).

    I love this setup. It gives me great performance for normal data and applications, along with a reasonable size storage for my media files. I also have a 4TB external USB drive for Time Machine and I also do Time Machine to my Synology NAS (2 backup locations)
     
  7. camner thread starter macrumors regular

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    #7
    Thanks much for your reply. I'm curious how you know that your setup (with the external TB SSD) results in faster access times than using the 2TB Fusion drive as your boot/Applications drive.
     
  8. bplein macrumors 6502

    bplein

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    #8
    For small things, you will tend to stay on the SSD portion of the Fusion drive, remember, you have 128GB (or is it 64 on the newer systems?) of stuff that's there. The OS isn't "pinned" there, it just happens to stay there due to frequent access. Similarly, if you don't use an app very often, it may move off to HDD. Also if your home directory is there, all of the data is there unless you proactively move it off.

    The OS tries to keep 4GB free on the SSD portion of a Fusion Drive if I recall correctly. So after you get it full, you WILL end up having background operations to read the SSD and move the data to HDD. Access times are reduced.

    I look at it this way: OS+Applications fits nicely in 500GB with room to spare for my user home directory. The largest collections of data many people have are iTunes (music, video) and pictures/media (non-iTunes video), which can easily be put onto a capacity-optimized drive. With the Fusion drive aspect, this capacity-oriented drive also gives good speed for sequential operations (great for moving images and video off an SD card for example) and it doesn't matter if your music is streaming from the HDD portion or the SSD portion. In fact, it could mostly be just HDD but I have no need for a small standalone 128GB SSD internal to the iMac, so I just left the Fusion drive as-is but deprecated it to use for sequential data.
     
  9. Fishrrman macrumors G4

    Fishrrman

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    Feb 20, 2009
    #9
    On a 2tb fusion iMac, you will ALWAYS achieve the FASTEST performance if you "un-fuse" the internal SSD and HDD, and then run them as "standalone" drives.

    Put the OS, apps, and basic accounts on the internal SSD.
    Keep large libraries of pics, movies and music on the HDD.

    Keep the internal SSD "lean and clean" that way.

    If you do, it will run considerably faster than ANY SATA or m2 SSD is capable of.
    USB3 or thunderbolt (2) simply can't deliver those speeds.

    That's "the fastest way"...
     
  10. casperes1996 macrumors 65816

    casperes1996

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    #10
    Well, their theoretical bandwidth is that high. With USB 3.0 the latency would be a lot higher, but TB2 should be able to achieve pretty much identical performance to an internal PCIe x4 connection. The performance difference there'd be around 1% if the SSD used was of exactly the same speed. Thunderbolt is essentially just external PCIe.

    I also disagree with the un-fusing thing. You're right in that you have full control over what gets the speed and what doesn't but I find that macOS/OS X already does tremendously well when it comes to managing the data on the drives, so it's not necessary.
     
  11. ApfelKuchen macrumors 68030

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    #11
    I have two late 2013 iMacs - one with 3TB Fusion, the other with pure Flash. In day-to-day use it's very hard to tell the two apart, performance-wise (yeah, I could benchmark...). I think trying to out-smart a computer-managed cache is a pointless exercise, unless you design caches for a living (I don't).

    I have one, concrete recommendation for you: Unless you need more than 2 TB of storage, try running on that internal Fusion alone for a while, and see how it works. I think you may be surprised at how well it does the job. I'd put OS, all apps, and any regularly-accessed data on the internal Fusion, infrequently-accessed data on the external. I know, this may completely waste the potential of that external SSD...

    Now, it may turn out that configuring the external SSD as the boot drive with OS and apps, and the internal Fusion for "slow" storage may work even better - in theory the Thunderbolt interface shouldn't slow things down. HOWEVER, if that external SSD is a SATA drive, it's at a permanent speed disadvantage to the Flash portion of the Fusion drive. Would you rather have your apps and data in Flash, or SATA? The internal "slower" Fusion Drive could turn out to be faster than the external SSD in practical use.

    As of today, you cannot use an external drive for Fusion. Otherwise, it might be potentially cool to fuse that external 500 GB SSD with the 2 TB HDD (then again, if it's a SATA drive, maybe it won't be so cool). I think Apple is smart to not provide the capability. Running a boot drive off of external is scary enough (been there, done that, hate the idea that "tripping" over an external wire/pulling the wrong cable can bring down the machine), but breaking a Fusion drive that way is even more perilous, as your data is spread, non-redundantly, across multiple drives. (Backup is even more essential with Fusion than with conventional drives.)

    ------

    When folks here discuss Fusion, I see many misconceptions about how these things work. Most important, for these purposes... Fusion works with blocks of data/code, not entire files, folders, apps, libraries, etc., essentially the same way blocks are read/written to/from RAM.

    The folder/file/library metaphors constantly trip us up in regard to Fusion. If we were managing a split SSD/HDD configuration manually, we would have to deal with entire apps and data structures (like Photos and iMovie libraries), so we may think in those terms when we imagine how Fusion works (all 4.04 GB of FCPX, or my entire 365 GB Photos library being moved from HDD to Flash). However, the OS, apps, and even individual data "files" are highly modular under the hood - files and folders protected within a "package" (folder with special properties). We can't easily break those pieces up and divide them between HDD and Flash, but the OS can, and does, under Fusion. Unused portions of apps and OS live on HDD, so more data lives on Flash drive.

    -------

    Fishrrman and I always disagree about the performance of Fusion. The only thing true about his assertion, in my opinion, is that anything he stores in Flash will, naturally, perform at full Flash speed.

    He constantly ignores the fact that everything on the HDD can perform only at HDD speeds, as if data or less-frequently used apps do not benefit at all from faster storage. Perhaps he only plays streaming media from his HDD. In that case, yes, there's no real benefit - read once from HDD, and it's gone. Perhaps he has so much RAM that there are no further reads from HDD after he opens a data file.

    My Fusion Drive has 128GB Flash. That's as much Flash as an entire base model MacBook Air. The vast majority of the code and data I work with on a daily basis resides in Flash, automatically. Therefore, most of the time, I pay no "penalty" at all. If I shut down my system and start it up tomorrow, whatever data I was working on today (and probably for many weeks into the past) is still going to be in Flash. I only pay a speed penalty when code/data is brought in from HDD or the cloud, and that's a one-time penalty, since afterwards, it's in Flash, automatically. I have a 365 GB Photos library that opens, if you excuse the expression, in a flash (because the entire library doesn't have to be Flash-resident to open quickly). The far smaller Photos library on my father's newer, all-HDD iMac doesn't open nearly as fast.

    Fusion is not about achieving maximum, theoretical performance from only those items that reside in a limited amount of Flash. It's about using Flash and caching to greatly improve the performance of a large, lower-cost HDD. If that premise didn't work, then no argument. In my experience, it works beautifully. With no management. No, "I'm running out of Flash storage, what should I move to the HDD?" Further, it uses all 128 GB of that Flash to my advantage. If, say, OS and apps occupied 64 GB of the Flash drive, would I then spend time figuring out how to exactly fill the remaining 64 GB, or might I leave part of that 64 GB unused, for overhead? I likely have far more stuff operating at Flash speeds than I'd every have by managing it manually.

    It's a whole lot like the stick shift vs. automatic transmission debate. Driving stick shift can be fun at times, and if you're good at it, you can get up to highway speeds more efficiently than with an automatic transmission. But it's quite a chore in stop-and-go traffic and general urban/suburban driving. I drove stick for decades but currently drive an automatic (Honda Civic). I'm really pleased with the performance of today's automatics - no significant gas mileage penalty, very responsive... overall, a pleasure to drive, even when accelerating to highway speeds.
     
  12. casperes1996 macrumors 65816

    casperes1996

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    #12
    I just want to note that I strongly agree with this post

    However, I was pretty certain you could create Fusion Drives with external media? Are you positive this isn't possible? Do you mean only that you can't mix and match internal and external, or also that you can't create a Fusion Drive with two external drives, an HDD and an SSD?
     
  13. bplein macrumors 6502

    bplein

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    #13
    You can mix and match internal and external drives for Fusion devices. That said, I'd probably keep the Fusion drive internal. External drives are more subject to ejection via knocking a cable loose. If you corrupt the Fusion drive and cannot put it back together, you lose all the data and have to revert to backups. Also remember that if an external SSD is USB3, then you aren't guaranteed that TRIM will work (although some chipsets support it).

    I use Thunderbolt on my 2012 for boot SSD and Fusion internal for bulk data.
    --- Post Merged, Mar 1, 2017 ---
    ALWAYS meaning assuming it's 2TB with PCIe flash. If it's SATA Flash, then the answer is that the internal is not any faster than the external.

    "ALWAYS except when it's NOT" (grin)

    I assume you mean in the OPs situation, but I have a 2TB Fusion (not original) in my 2012 and the internal SSD is no faster than the external. And 128GB is really small for some users, may be good enough for you but not for me.
     
  14. ApfelKuchen macrumors 68030

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    #14
    From Apple's Fusion FAQ https://support.apple.com/HT202574
    Now, people have been able to hack it (statements like that from Apple tend to throw the gauntlet)... but officially, no.
     
  15. bplein macrumors 6502

    bplein

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    #15
    If by saying "using the standard CLI to do something that's not supported by the vendor" is "hacking", yes, it's a hack. But it's completely "standard" to use the CLI to build a Fusion Drive. The reason Apple doesn't want you to use external drives is probably related to my prior comment about the possibility of corruption due to cable loss, flaky enclosures, bad SATA chipset on the far end of the TB or USB link, etc.
     
  16. casperes1996 macrumors 65816

    casperes1996

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    #16
    Yeah, but in general the official Apple Core Storage commands used to manipulate Fusion Drives aren't supported either. They are made by Apple, but not supported by Apple for end-user usage.
    What constitutes a hack in this case I guess isn't rigidly defined, but in this case, I never expected anything but that you would have to use the CLI
    --- Post Merged, Mar 1, 2017 ---
    Right, thought so too.
    As far as I've heard, no Mac supports UASP and therefore TRIM for external drives. There's another thread where I've discussed this subject with a few people, including @Weaselboy who's a staff member here, who's tested it himself on I believe it was a 2016 MacBook Pro with USB 3.1 and an enclosure said to support UASP.
    For the cable reasoning, I wouldn't necessarily recommend mixing internal and external drives for a Fusion, but if you keep backups, and you have a stable, none-changing setup, I'd personally be willing to take the risk.
    Assuming no writes are going on, if you accidentally knock the cable out, presumably crash the system, plug back in the drive, and reboot, I assume it would still function, since all the data should be intact across the two drives, since no changes occurred when the connection was dropped?
     

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